In view of the increased demand for energy supply several thermochemical processes for the production of hydrogen have been recently investigated. Common to almost all of these closed-cycle multi-step processes is the thermochemical decomposition of water into hydrogen and oxygen, simultaneously using various auxiliary compounds especially inorganic metallic salts, e.g., iron or copper compounds, and anions, e.g., sulfur or halides. However, the discovery of suitable closed-cycle processes, which mostly comprise four or more different steps and at the same time are technically and economically feasible, is rather difficult. Therefore out of the manifold of theoretically possible processes only a few practical reaction sequences have been selected and described, especially those using the systems: alkaline earth-mercury-bromine, iron-chlorine, chromium-chlorine, manganese-chlorine and iron-sulfur. Until now only a few specific combinations of certain reaction mechanisms have been proposed for decomposing water into its elements.